Although hydrocarbon fuels remain as the dominant energy resource for internal combustion engines, alcohols such as methanol and ethanol have also been used as fuels. Ethanol, the primary alcohol fuel, is commonly blended into gasoline in quantities of 5 to 10%. In fact, various fuels being produced today consist primarily of alcohols. For example, E-85 fuel contains 85% ethanol and 15% gasoline, and M-85 fuel has 85% methanol and 15% gasoline. While ethanol possesses excellent octane enhancement properties, there are several drawbacks to its use as a gasoline component, including: energy deficiencies (ethanol provides approximately 39% less energy than gasoline), high blending Reid Vapor Pressure (RVP) (at 10% of blending, the RVP=11 psi), and incompatibility with existing transportation facilities.
Previously, lead (Pb) was added to gasoline to increase its octane rating and thereby improve its antiknock properties. However, the use of lead in gasoline has now been eliminated in most countries for health and environmental reasons. Consequently, methyl-tiary-butyl-ether (MTBE) was commercially introduced as an octane enhancing component of gasoline in the United States and other countries in the late 1970s in order to meet the need for increased octane ratings in the absence of lead. Legal restrictions on the minimum oxygen content of some gasolines—introduced in the 1990s as a means of reducing environmentally harmful exhaust emissions—encouraged a further increase in the concentration of MTBE in gasoline, which, by then, was being blended at up to 15% by volume. While MTBE is still widely used in some countries such as the United Kingdom, its use has been in gradual decline in other regions of the world due to concerns about the harmful effects of MTBE itself. Specifically, its existence in groundwater has led to a decline in its use in the United States, where some states have actively legislated against its use. Thus, to meet today's performance and legal requirements, the fuel industry in the United States is now replacing MTBE with fermented grain ethanol. Producing the necessary quantities of grain ethanol to replace MTBE, however, has proven problematic in specific regions, and the use of ethanol as a gasoline component has other drawbacks as discussed above.
Certain other alcohols (i.e., butanols), as well as butene oligomers (e.g., diisobutenes (DIBs)) can be used as combustible neat fuels, oxygenate fuel additives, or constituents in various types of fuels. The BTU content of butanols and diisobutenes is closer to the energy content of gasoline than either ethanol or methanol. Butanols have been thought of as second generation fuel components after ethanols. In particular, 2-butanol and t-butanol can be advantageous fuel components, as they have blending octane sensitivities and energy densities comparable to those of MTBE and have been shown to have lower RVP at 15% concentrations relative to comparable ethanol blends. Likewise, DIB is a non-oxygenated fuel component with several advantages over other fuel additives. For instance, DIBs have better anti-knock quality, higher RON, and higher energy content compared with MTBE, as well as a lower RVP than ethanol, butanols, or MTBE.
Butanols can be produced via the hydration of butenes, a process that typically utilizes an acid catalyst. While the production of butanols via hydration of butenes is a commercially important process, it is typically very costly. DIBs are produced via the oligomerization/dimerization of butenes, in particular isobutene. The dimerization of isobutene is also generally performed using acid catalysts, such as sulfuric acid and hydrogen fluoride; however, these catalysts tend to be highly corrosive in nature.
Butanols and DIBs provide certain advantages over other existing fuel components. For instance, the combination of butanols and DIBs as a fuel additive would lead to enhanced octane sensitivity, energy density, and RON, as well as decreased RVP in gasoline. Only recently have there been any processes for converting mixed olefins into alcohols—especially butenes into butanols—while also dimerizing part of the mixed olefins feed into oligomers such as DIBs without requiring the costly separation of either mixed butenes isomers in the feed or the mixed butanol isomers in the product. Still, inefficiencies exist in current processes for the contemporary hydration and dimerization of mixed butenes.
U.S. Patent Application No. 2013/0104449 (the '449 publication) is a method for contemporaneously dimerizing and hydrating a hydrocarbon feed containing butene, resulting in the production of alcohols and DIBs. While the '449 publication addresses the need for simultaneously hydrating and oligomerizing a mixed butene feed to produce butanols and DIBs, the process in '449 publication is limited in its ability to 1) separate the butanol/DIB products from water, 2) produce high-purity, on-specification butanols, and 3) convert butenes to butanols and DIBs in a single pass. These limitations lead to increased costs, specifically the costs associated with recycling unreacted butenes back to the reactor to increase product yield and costs associated with further separating the product from water to produce higher purity butanols and DIBs.
Thus, there is a need for alternative gasoline oxygenates that possess comparable RON enhancement properties and a higher energy content than MTBE and ethanol, but that also eliminates the environmental and compatibility concerns of MTBE and ethanol. There is also a need for alternative fuel additives that lower the RVP of fuel in the absence of MTBE. Finally, there is a need for a process that not only allows for the contemporaneous hydration and oligomerization of mixed butenes to alcohols and oligomers—namely butanols and DIB—but also increases the conversion rate of butenes and produces higher purity product streams.